Ah I see what you mean, yeah, you could just download it even if you have the disc(which is a nice option if you lose the disc!). Personally, i don't like steam for large games(mostly due to download speeds and poor server uptimes for big launches), it's great for small indie games though.
For me, the issue is because when you do that, like with how Blizzard is doing Diablo, is that you're basically renting the game. To have to be connected to THEIR servers be reliant on THEIR uptime means that YOU don't have control over your playtime, at least not in the same way you would as games that never need to be connected after registration. It also means that you have to be paying for 24/7 internet service, which is expensive and if you need to cut your personal costs or move to a new area and have some time without internet or heck, if the ISP is down for a while(common in my area).I don't really see the issue with steam DRM even if you had to be connected to the internet to play, it;s not like I will be playing whilst travelling.
Basically the short answer is: constant internet connection takes control out of the hands of the owner and makes them reliant on the services of others. At least there was telephone verification for people w/o internet for registration...but requiring a 24/7 connection for a game w/no regular online component(like Diablo 3's AH) is not right IMO.